There are a variety of situations in which it is desired to expand a metal tube radially to form a tight, leak-proof joint. For example, large heat exchangers, particularly the type used as steam generators in nuclear power plants, often employ a tube sheet, which is a metal plate several feet in thickness through which hundreds of stainless steel or carbon steel tubes must pass. The tube sheet is fabricated with through bores of a suitable diameter in which the tubes are inserted. The tubes are then expanded against the sides of the bores by plastic deformation to seal the small crevices that would otherwise exist around the tubes. If these crevices were allowed to remain, they could collect corrosive agents, and would, therefore, decrease the predictable life-expectancy of the equipment.
Older techniques for expanding the tubes to form the desired leak-proof joints relied upon roller swaging. However, mechanical rolling of the interior surface of the tube causes a decrease in the thickness of the tube wall. In addition, roller swaging is a time-consuming process and it is sometimes difficult or impossible, particularly in the case of small diameter tubes, to obtain the swaging pressures desired.
More recently, superior tube and tube sheet joints have been formed by hydraulic swaging. In accordance with this technique, a mandrel is inserted in the tube and a pressurized working fluid is introduced through the mandrel into a small annular space between the mandrel and the tube. The fluid is axially confined between seals and applies high outwardly directed radial pressure to the tube wall.
O-rings are usually used for the seals. In the case of high-pressure applications, it is desirable to use O-rings in combination with back-up members of a stiffer material such as polyurethane, as explained in this inventor's co-pending application, Ser. No. 133,013 filed on Mar. 24, 1980, and entitled SELF-CENTERING SEAL FOR USE IN HYDRAULICALLY EXPANDING TUBES now U.S. Pat. No. 4,359,889, issued Nov. 23, 1982.
O-rings employed in this environment must have a sufficient diameter and rigidity to effectively confine the hydraulic fluid in the desired manner. When an O-ring of suitable size and properties is inserted in a tube it offers very high frictional resistance, binding against the interior tube surface. Insertion of the mandrel is therefore difficult and time-consuming. Remembering that large numbers of tubes are often installed in a single tube sheet, the difficulties attributable to frictional O-ring resistance to mandrel insertion is a major factor bearing upon the efficiency and effectiveness of hydraulic swaging techniques that have been employed.
A principal objective of the present invention is to provide a swaging apparatus and method for forming joints between tubes and tube sheets in which the resistance offered by the seals as the mandrel is inserted in the tube is greatly reduced, although the effectiveness of the seals is not diminished.